1. Field of Invention
The present invention pertains to the field of automatic transmissions for motor vehicles and, more particularly, to a method and system for providing a Brake Transmission Shift Interlock Override mode in a shift-by-wire transmission.
2. Background of the Invention
A traditional automatic transmission includes a transmission control device employed to control the transmission of a motor vehicle. The transmission control device is used to select several ranges, such as Park, wherein the transmission is locked to prevent the vehicle from moving, Neutral, wherein the transmission allows the vehicle to be moved freely, such as when being towed, Reverse, wherein the transmission allows the vehicle to move backwards, and one or more Drive ranges that enable forward motion of the vehicle. Usually, the transmission control device is in the form of a lever connected with a mechanical connection, such as a cable or a hydraulic line, to the transmission. Typically, the lever is also connected to an indicator. As the transmission control mechanism is moved from one range to another, the mechanical connection physically shifts the transmission to the selected range and the indicator moves to show the driver which range has been selected. Even if the vehicle is turned off, the driver is able to determine the current transmission range from the indicator and, in some cases, is able to move the transmission control mechanism to Neutral if, for example, the vehicle is to be towed.
The traditional automatic transmission utilizes multiple friction elements for automatic gear ratio shifting. Broadly speaking, these friction elements may be described as torque establishing elements although more commonly they are referred to as clutches or brakes. The friction elements function to establish power flow paths from an internal combustion engine to a set of vehicle traction wheels. During acceleration of the vehicle, the overall speed ratio, which is the ratio of a transmission input shaft speed to a transmission output shaft speed, is reduced during a ratio upshift as vehicle speed increases for a given engine throttle setting. A downshift to achieve a higher speed ratio occurs as an engine throttle setting increases for any given vehicle speed, or when the vehicle speed decreases as the engine throttle setting is decreased. Various planetary gear configurations are found in modern automatic transmissions. However, the basic principle of shift kinematics remains similar. Shifting an automatic transmission having multiple planetary gearsets is accompanied by applying and/or releasing friction elements to change speed and torque relationships by altering the torque path through the planetary gearsets. Friction elements are usually actuated either hydraulically or mechanically based on the position of the transmission control device.
In a shift-by-wire transmission arrangement, the mechanical connection between the transmission control device and the transmission is eliminated. Instead, the transmission control device transmits an electrical signal along a wire to an electronic controller, which directs separate actuators to apply or release the various friction elements to obtain a desired gear ratio. The control device is no longer necessarily in the form of a lever because the control device is no longer moving a mechanical connection for controlling the transmission. Instead, the control device is typically an electro-mechanical interface (e.g., series of buttons, lever, knob) that are used to instruct the transmission to switch between the transmission ranges. An electronic display, powered by a battery on the vehicle, is typically employed to indicate the current range for the transmission and must be on, and thus drawing power, in order for the driver to know which range has been selected.
Many vehicles with a shift-by-wire transmission incorporate a “Return to Park” feature to automatically shift the transmission into Park. See, for example, U.S. Pat. Nos. 3,937,105, 4,892,014 and 7,156,218. Such a feature is activated when certain triggering events occur, for example, when the system detects a seat belt being unbuckled while a driver door is opened when the vehicle is traveling below a calibrated speed, or when the ignition is turned off. Automatically shifting the transmission into Park prevents unwanted motion of the vehicle.
Transmissions often include a brake transmission shift interlock so that once the transmission is shifted into Park the interlock does not allow the transmission to be shifted out of Park unless the ignition is on and the brake pedal is depressed. In a shift-by-wire transmission, in addition to the brake pedal being depressed, the transmission control device must be powered and a vehicle key or fob must be present. While such a feature is helpful in preventing unwanted vehicle movement, once again such a feature is not helpful when the vehicle is to be towed or moved urgently when the vehicle key or fob is not present or when the transmission control device is not functioning properly.
As can be seen by the above discussion, there is a need in the art for a system that allows the vehicle to be shifted from Park to Neutral and maintain Neutral, without the presence of a key or in the event of a brake sensor failure. In a conventional mechanical shifter, Neutral is typically provided by accessing an over-ride lever through a removable panel. Actuating the lever allows the shifter and transmission to be moved from Park into Neutral. With a shift-by-wire transmission, the need for providing a feature that allows the vehicle to shift from Park to Neutral and maintain Neutral without the presence of a key is desired for towing the vehicle for service. This same feature would also allow a shift from Park to any other range in the event of a brake sensor failure, allowing the vehicle to be driven until the brake sensor can be serviced.